Rosin is derived from pine trees, chiefly Pinus palustris and Pinus caribaea. Gum rosin is the residue obtained after the distillation of turpentine oil from the oleoresin tapped from living trees. Wood rosin is obtained by extracting pine stumps with naphtha and distilling off the volatile fraction. Tall oil rosin is a co-product of the fractionation of tall oil. The chief constituents of rosin are the resin acids of the abietic and pimaric types, having the general formula C.sub.19 H.sub.29 COOH and having a phenanthrene nucleus.
The esterification of rosin is well known in the art. Such esterified rosin products are used extensively for preparing adhesives, coatings, ink varnishes and other materials.
Rosins are generally esterified by the thermal reaction of the rosin with an alcohol such as a polyol. A polyol in widespread use today is pentaerythritol, with four active hydroxyl groups. Tall oil rosin is widely used as a source of rosin for its availability and low cost.
The rosin ester market of today requires lighter colored products than are currently available. The known methods to produce lighter colored rosins are expensive since they result in rosin quality improvements at the expense of rosin yield. Such methods include the use of solid adsorbents such as carbon, esterifications in solvent, or the use of hydrogenation of the ester. Each of these techniques, while producing rosin esters with desirable characteristics, are time consuming and add unacceptable cost of the final product.
It is known to use small amounts of calcium hypophosphite to catalyze the reaction of a rosin with a polyol to form the ester. This reaction produces esters with colors in the range of 5+ on the Gardner scale. Such an ester is an improvement over the 6- on the Gardner scale produced by the esterification of the rosin without the calcium hypophosphite.
It is also known to use hypophosphorous acid to catalyze the rosin esterification reaction to produce lighter esters. Like the calcium hypophosphite catalyst, the hypophosphorous acid produces rosin esters with a color improvement of about one-half a number on the Gardner scale. The one-half number improvement is about the maximum improvement seen with either calcium hypophosphite or hypophosphorous acid.
Despite the above improvements, there is a growing demand for still lighter rosin esters. The lightening of a rosin ester by as little as an additional one-half a Gardner color number (that is, to a color of 5- or less) is highly desired in the marketplace. Such rosin esters would be used to prepare a variety of more desirable, lighter colored adhesives.
It is, therefore, an object of the present invention to provide a new method for the production of rosin esters which would yield rosin esters having colors lighter than about 5 on the Gardner scale.
It is another object of the present invention to provide such a process which produces rosin esters suitable for use in adhesives.
It is a further object of the invention to provide rosin esters which have Gardner colors of about one number less than the rosin esters prepared without additives.